A variety of techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), pyrolysis and similar chemical conversion processes, anodizing, electrostatically charged powder deposition, and thermal spraying (including flame spraying, high-velocity oxy/fuel spraying, and plasma spraying) are commonly used to deposit dielectric, ceramic, and semiconductor coatings. Applications for these coatings are in corrosion protection, thermal management, optics, and electronics.
For example, aluminum and its alloys are commonly anodized to form aluminum oxide coatings that slow salt water spray-induced corrosion of machinery and architectural elements. Anodized aluminum alloy plates and metal plates with thermal-spray electrical insulators are used as supports to hold solar cells wired in series.
Many photovoltaic mounting structure designs specify that the electrically insulating coating must have good thermal conductivity so that the cells can be cooled efficiently. It is a common practice to anodize satellite hardware to control optical emissivity. The semiconductor fabrication industry uses anodized aluminum fixtures in plasma-assisted etch and CVD tools to protect these parts against corrosive working gases, and shape plasmas or tailor plasma potentials. Anodic coatings and thermal-spray coatings are used as dielectrics on electrostatic chucks to hold electrically conductive parts during fabrication or processing.
The dielectric, ceramic, optical, and semiconductor coatings that are applied by PVD, CVD, chemical-conversion processes, anodizing, and thermal spraying may be porous, cracked, or flawed, permitting corrosive liquids, gases, and vapors to attack the underlying substrates. Pores, cracks, and flaws may give rise to anomalies in, or totally dominate, the electrical properties of these coatings, or increase electrical leakage and reduce electrical-breakdown strength. Pores, cracks, and flaws reduce thermal conductivity, and can harbor gases, liquids, and vapors that add to the gas load if these coatings are used in a vacuum system.
It is common practice to seal pores in anodic aluminum oxide coatings by immersing anodized parts in water at or near the boiling point, or by processing the parts in an autoclave. The anodic aluminum oxide is thus hydrolyzed and converted to boehmite which seals the pores. The amount of boehmite formed by hydrolyzing anodic aluminum oxide is sufficient to fill the pores in a coating to some depth, but it does not adequately seal relatively large cracks and defects. Boehmite is mechanically and chemically fragile compared with many sol-gel derived ceramics, and has an index of refraction and optical absorption bands which may not be desirable in optimizing the optical properties of a coating.
High-velocity oxy/fuel, plasma-spray processes, and vacuum plasma-spray processes can be used to deposit relatively dense coatings. (For certain applications, it is desirable to have some amount of porosity at the coating/substrate interface of a thermal-sprayed coating to accommodate mismatches in thermal coefficient of expansion between the coating and the substrate.) These techniques require expensive equipment that is beyond the economic resources of many commercial thermal-spray coating facilities.
There are no techniques that are commonly used for filling, sealing, or densifying PVD coatings or pyrolytic and similar conversion coatings, with the exception of pyrolytic and conversion coatings used for decorative purposes. Chemical-conversion coatings used decoratively, such as patinas, are usually sealed with wax or shellac.
Electrophoresis is movement in a solution or a dispersion of charged molecules or particles under the action of an applied electric field. During electrophoretic coating deposition, charged particles in liquid suspension migrate toward, and deposit on, an oppositely charged conductive electrode which may be either the cathode or the anode, depending on particle charge; for the particular materials described as examples in the present invention, the coating substrate is cathodic. Electrophoretically deposited coatings have many practical advantages that have led to their commercial use. For example:
1. many different materials can be made electrophoretically active and deposited on conductive substrates, PA1 2. coating thickness can be readily controlled, PA1 3. thick coatings (order of millimeters) can often be applied, PA1 4. two or more materials can often be co-deposited, PA1 5. coating occurs rapidly, and PA1 6. scale-up to production is straightforward.
Deposition rate decreases with time due to the increasing electrical resistance of the growing film during constant-voltage electrophoretic deposition. Since film deposition is enhanced in defective regions of the growing film where the electric field is the highest, pinhole-free films of uniform thickness can be deposited on surfaces of even complex shape.
U.S. Pat. No. 4,357,222 describes an electrophoretic casting process which produces highly dense green castings with residual liquid (water) below 7%.
U.S. Pat. No. 4,971,633 describes a thin, porous, Al.sub.2 O.sub.3 film, used in solar cells, filled with an electrophoretically deposited layer of a styrene acrylate resin.